The present invention relates to an improved method and apparatus for measuring the body fat of a live subject, and more particularly, of a living human being.
Body composition, and in particular percent body fat, is a well-recognized measure of physical health. Various techniques of determining percent body fat are presently in use, including caliper measurements, underwater displacement measurement, and bioelectrical impedance measurement.
In this last technique, as described in U.S. Pat. Nos. 5,415,176 and 5,611,351, both to Sato et al., a live subject whose body fat is to be measured stands upon a scale-like device with four electrodes mounted on its upper surface. A 50 kHz, 800 microampere electric current is produced by constant current source. This current is passed first through two electrodes in contact with the subject""s toes and then through two reference resistors located in series with the subject""s body and with each other. The electric current flowing through the subject causes a voltage potential to develop across the subject""s heels.
Using a microprocessor-controlled switch array and a voltage measuring circuit, the heel-to-heel voltage is measured via two other electrodes in contact with the subject""s heels. The voltages across the reference resistors are also measured while the electric current is applied to the subject""s toes. A comparison of the voltages measured across the reference resistors with the heel-to-heel voltage provides a highly accurate measure of the heel-to-heel impedance. After certain additional parameters such as age, weight, and height are entered into the microprocessor, it calculates body density using an algorithm relating body impedance and the additional parameters to body density. Once body density is obtained, the microprocessor performs a second calculation to convert body density to percent body fat.
The present inventors have identified several disadvantages of the device taught by Sato et al. First, the 800 microampere current produces a relatively strong electric field that is centered on the current-supply electrodes. This field distribution restricts the location of the current-supply electrodes with respect to the voltage-detecting electrodes: the two sets of electrodes are required to be at least 5 cm apart. This distance limitation can present a problem in the body fat measurement of small children.
Second, the electrodes in the device of Sato et al. are flat and quite large, to accommodate a range of adult feet sizes. The pressure of the subject""s weight on his feet, which are in contact with the wide, flat surface of the electrodes, restricts blood flow to the tissues above the electrodes. It is believed that this constriction causes the measured body impedance to be artificially increased, creating a source of error in the body fat measurement. Furthermore, the large electrodes of the device of Sato et al. causes the equipment to appear clinical and unaesthetic, and not at all user-friendly.
Third, the reference resistors in the device of Sato et al. are placed in series with each other and with the subject""s body. This configuration limits the device""s durability and reliability, for if the connection between any of the resistors or the current supply electrodes is accidentally broken, the device becomes entirely unable to function. In addition, the device is only capable of measuring combinations of reference resistors which are adjacent to each other. This arrangement thus limits the resolution of the measurement.
Finally, the current-supply electrodes in the device of Sato et al. contact the subject""s toes, while the voltage-measurement electrodes contact his heels. Thus, the current flows from the toes through the feet toward the heels and then up through the legs. The measured body impedance thus includes the toe-heel impedance of each foot. Because toe-heel impedance is not included as an independent variable in most equations correlating body impedance and percent body fat, however, variations in foot size and foot impedance from subject to subject introduce additional error in the body fat calculation.
Accordingly, it is an object of the invention to overcome these and other problems associated with the prior art, and to provide an accurate, robust, high-resolution body fat analyzer that may be used by a wide range of subjects.
In accordance with one embodiment of the present invention, a subject""s body composition is measured by (1) supplying a 50 kHz, 300 microampere electrical current to the subject""s body via a group of current-supply electrodes that contact the subject""s heels, (2) measuring the voltage across a group of voltage-detecting electrodes that contact the balls (and/or toes) of the subject""s feet and across a plurality of internal reference resistors connected in parallel with the subject""s body, and (3) calculating from these measurements the body fat percentage as a function of body impedance.
A preferred embodiment of the invention comprises a current source connected in parallel with two or more reference resistors and with the subject""s body. The resistors and the subject""s body are switched in and out of the circuit, and the various voltages across the resistors and the body are detected by a differential amplifier. The output of the differential amplifier is conditioned by a rectifier and low-pass filter and input to an analog-to-digital convertor (ADC). The output from the ADC is presented to a microprocessor control unit, which calculates (1) the impedance of the individual""s body based upon the various voltage measurements, and (2) the percent body fat as a function of that impedance and other variables such as height, weight, age, and sex.
Additionally, the electrodes in the present invention are preferably designed as an array of small round knobs raised slightly above the surface upon which they are mounted. The electrodes are grouped into four groupsxe2x80x94two current-supply electrode groups that contact the right and left heels, and two voltage-detecting electrode groups that contact the balls of the feet. Because a current of only 300 microamperes is preferably used, rather than the more conventional 800 microamperes, the current-supply electrodes and the voltage-detecting electrodes may be quite close to each other. In a preferred embodiment, the electrodes in the current-supply group are separated from the electrodes in the voltage-detecting group by a distance as small as about 1 cm.
This electrode array configuration has a number of advantages over the prior art. First, the electrodes may be spaced sufficiently close together to allow even small-footed children to use the device. Second, the subject has great flexibility as to the specific location of his feet on the device. Lastly, since the subject""s foot makes contact with a number of small electrodes distributed over the surface of the foot, the blood circulation in each foot is enhanced, as compared with the flat plate electrodes as described in the background above, and the measurement is rendered thereby more reproducible.